learnxinyminutes-docs/scheme.html.markdown

---
language: "Scheme" 
filename: scheme.scm 
contributors:
- ["Bruno Ciccarino", "https://github.com/BrunoCiccarino"]
---

Scheme is a minimalist dialect of Lisp that is widely used in education, research, and industry. It emphasizes simplicity, powerful abstractions, and functional programming paradigms.

A classic resource to learn Scheme is [Structure and Interpretation of Computer Programs (SICP)](https://web.mit.edu/6.001/6.037/sicp.pdf). For a modern introduction, consider [The Scheme Programming Language](https://www.scheme.org/).

```scheme
;;;-----------------------------------------------------------------------------
;;; 0. Syntax
;;;-----------------------------------------------------------------------------

;;; General form

;;; Scheme has two fundamental elements of syntax: ATOM and S-EXPRESSION.
;;; S-expressions are used for both data and code.

10          ; a number atom; evaluates to itself
'symbol     ; a symbol atom; evaluates to itself when quoted
#t          ; boolean true
(+ 1 2 3)   ; an s-expression (function application)
'(4 'foo #t) ; quoted s-expression (a list)


;;; Comments

;;; Single-line comments start with a semicolon:
; This is a single-line comment

;;; Block comments use `#|` and `|#`:
#| This is a block comment.
   It spans multiple lines.
|#


;;; REPL and environment

;;; Scheme is typically developed interactively in a Read-Eval-Print Loop (REPL). 
;;; Implementations such as Racket, Guile, or MIT Scheme provide REPLs for interactive exploration.
;;; Libraries and tools can be installed depending on the specific implementation.



;;;-----------------------------------------------------------------------------
;;; 1. Primitive datatypes and operators
;;;-----------------------------------------------------------------------------

;;; Numbers

42          ; integers
#b101       ; binary => 5
#o777       ; octal => 511
#xFF        ; hexadecimal => 255
3.14        ; floating-point numbers
1/2         ; fractions (exact rational numbers)
(make-rectangular 1 2) ; complex numbers


;;; Basic arithmetic

(+ 1 2)     ; => 3
(- 7 3)     ; => 4
(* 2 5)     ; => 10
(/ 10 3)    ; => 10/3
(sqrt 4)    ; => 2
(expt 2 3)  ; => 8


;;; Booleans

#t          ; true
#f          ; false
(and #t #f) ; => #f
(or #t #f)  ; => #t
(not #t)    ; => #f


;;; Strings

"Hello, World!"
(string-append "Hello, " "World!") ; => "Hello, World!"


;;; Lists

'(1 2 3)      ; a list
(cons 1 '(2 3)) ; => '(1 2 3)
(car '(1 2 3))  ; => 1
(cdr '(1 2 3))  ; => '(2 3)
(append '(1 2) '(3 4)) ; => '(1 2 3 4)


;;;-----------------------------------------------------------------------------
;;; 2. Variables
;;;-----------------------------------------------------------------------------

;;; Define a variable

(define x 10)
x           ; => 10


;;; Define a local variable

(let ((x 5)) (+ x 10)) ; => 15
x                      ; => 10 (unchanged globally)


;;;-----------------------------------------------------------------------------
;;; 3. Functions
;;;-----------------------------------------------------------------------------

;;; Define a named function

(define (square x)
  (* x x))

(square 4) ; => 16


;;; Define an anonymous (lambda) function

((lambda (x) (* x x)) 5) ; => 25


;;; Higher-order functions

(define (apply-twice f x)
  (f (f x)))

(apply-twice square 2) ; => 16


;;;-----------------------------------------------------------------------------
;;; 4. Conditionals and control flow
;;;-----------------------------------------------------------------------------

;;; If statements

(if (> 5 3)
    'yes
    'no)     ; => 'yes


;;; Cond expressions (multi-branch conditionals)

(cond
  [(< 5 3) 'less]
  [(> 5 3) 'greater]
  [else 'equal]) ; => 'greater


;;;-----------------------------------------------------------------------------
;;; 5. Structs and collections
;;;-----------------------------------------------------------------------------

;;; Define a structure

(define-struct dog (name breed age))

(define my-dog (make-dog "Fido" "Labrador" 5))

(dog-name my-dog)   ; => "Fido"
(dog-age my-dog)    ; => 5


;;;-----------------------------------------------------------------------------
;;; 6. Common patterns
;;;-----------------------------------------------------------------------------

;;; Recursive functions

(define (factorial n)
  (if (= n 0)
      1
      (* n (factorial (- n 1)))))

(factorial 5) ; => 120


;;;-----------------------------------------------------------------------------
;;; 7. Libraries and modules
;;;-----------------------------------------------------------------------------

;;; Importing libraries/modules depends on the implementation.
;;; For example, in Racket:

(require racket/math)

(sqrt 16) ; => 4

;;;-----------------------------------------------------------------------------
;;; 8. Macros
;;;-----------------------------------------------------------------------------

;;; Macros allow you to create new syntactic constructs.

(define-syntax when
  (syntax-rules ()
    [(when test body ...)
     (if test
         (begin body ...))]))

(when #t
  (display "Condition is true!\n")) ; Output: Condition is true!


;;;-----------------------------------------------------------------------------
;;; 9. Input and Output (I/O)
;;;-----------------------------------------------------------------------------

;;; Printing to the console

(display "Hello, Scheme!") ; => prints "Hello, Scheme!"
(newline)                  ; => moves to the next line


;;; Reading input

(let ((user-input (read)))
  (display "You entered: ")
  (display user-input))


;;; File I/O

(define output-port (open-output-file "example.txt"))
(display "Writing to a file." output-port)
(close-output-port output-port)

(define input-port (open-input-file "example.txt"))
(let ((file-content (read input-port)))
  (display file-content))
(close-input-port input-port)


;;;-----------------------------------------------------------------------------
;;; 10. Iteration
;;;-----------------------------------------------------------------------------

;;; Iterating with `do`

(do ((i 0 (+ i 1)))        ; initialize i to 0, increment by 1
    ((>= i 5))             ; stop when i >= 5
  (display i)              ; print i
  (newline))


;;; Using recursion for iteration

(define (countdown n)
  (if (= n 0)
      (display "Blastoff!\n")
      (begin
        (display n)
        (newline)
        (countdown (- n 1)))))

(countdown 5) ; Output: 5 4 3 2 1 Blastoff!


;;;-----------------------------------------------------------------------------
;;; 11. Error handling
;;;-----------------------------------------------------------------------------

;;; Using `guard` for error handling (Racket example)

(guard [e (displayln (format "Error: ~a" e))]
  (/ 1 0)) ; Output: Error: division by zero


;;; Catching exceptions manually

(with-handlers ([exn:fail? (lambda (e) (displayln "Caught an error!"))])
  (error "Something went wrong!")) ; Output: Caught an error!


;;;-----------------------------------------------------------------------------
;;; 12. Advanced concepts
;;;-----------------------------------------------------------------------------

;;; Continuations with `call/cc`

(call/cc
 (lambda (cont)
   (display "Before continuation\n")
   (cont #f)
   (display "After continuation\n"))) ; Output: Before continuation


;;; Lazy evaluation (streams)

(define (make-stream start step)
  (cons start
        (lambda () (make-stream (+ start step) step))))

(define nums (make-stream 0 1)) ; Infinite stream starting at 0, incrementing by 1

(define (stream-ref stream n)
  (if (= n 0)
      (car stream)
      (stream-ref ((cdr stream)) (- n 1))))

(stream-ref nums 5) ; => 5


;;;-----------------------------------------------------------------------------
;;; 13. Meta-programming
;;;-----------------------------------------------------------------------------

;;; Evaluate expressions dynamically

(eval '(+ 1 2)) ; => 3


;;; Quasiquoting for meta-programming

`(1 2 ,(+ 3 4)) ; => '(1 2 7)

```